Intake system of a multi-cylinder internal combustion engine

ABSTRACT

A multi-cylinder engine comprising a carburetor housing forming therein at least two branch mixture passages. Each of the branch mixture passages is connected to a respective intake port. A throttle valve of the carburetor is provided for each cylinder. Each of the throttle valves is arranged in the respective branch mixture passage and attached onto a common throttle shaft. A wedge shaped groove is formed on the bottom wall of each of the branch mixture passages, and the common throttle shaft is arranged in the wedge shaped groove for causing the mixture to flow only along the upper wall of the branch mixture passage. A single common connecting passage and branch connecting passages which are connected to the common connecting passage are provided. Each of the branch connecting passages opens into the respective intake port in the vicinity of the rear face of the valve head of the corresponding intake valve.

DESCRIPTION OF THE INVENTION

The present invention relates to an intake system of a multi-cylinderinternal combustion engine.

Particularly in a gasoline engine, in order to obtain a high outputpower of the engine by increasing the volumetric efficiency when theengine is operating at a high speed under a heavy load, the shape ofeach intake port is so constructed that the intake port has as small aflow resistance as possible. In the case wherein the intake port hassuch a shape, since a considerably strong turbulence is spontaneouslycreated in the combustion chamber of the engine when the engine isoperating at a high speed under a heavy load, the burning velocity issufficiently increased. However, when the same engine is operating at alow speed, a satisfactory strong turbulence is not created in thecombustion chamber, thus resulting in a problem in that a sufficientincrease in the burning velocity is not obtained.

As a method of creating a strong turbulence in the combustion chamberwhen an engine is operating at a low speed, there is a method ofcompulsorily creating a swirl motion in the combustion chamber by usinga helically-shaped intake port or by using a shroud valve. However, inthe case wherein such a method is adopted, since the flow resistance towhich the mixture fed into the cylinder is subjected is increased, aproblem occurs in that the volumetric efficiency is reduced when such anengine is operating at a high speed under a heavy load. In addition, asmeans of creating a strong turbulence in the combustion chamber, anengine has been proposed in which the intake passage comprises a mainintake passage having a relatively large cross-section and an auxiliaryintake passage having a relatively small cross-section and opening intothe intake port. In this engine, the mixture is fed into the combustionchamber from the auxiliary intake passage via the intake port when theengine is operating under light load; as a result, turbulence isproduced in the combustion chamber by the mixture spouted at highvelocity from the auxiliary intake passage. On the other hand, when thisengine is operating under a heavy load, the mixture is fed into thecombustion chamber via the main intake passage. Consequently, a strongturbulence is produced in the combustion chamber when the engine isoperating at a low speed under a light load, while ensuring a highvolumetric efficiency when the engine is operating at a high speed undera heavy load. However, this engine has drawbacks in that theconstruction of a mixture passage switching mechanism for switching themixture passage from the main intake passage to the auxiliary intakepassage is complicated; in addition, it is impossible to produce astrong turbulence in the combustion chamber when the engine is operatingat low speed under a heavy load.

An object of the present invention is to provide an intake system of aninternal combustion engine, which intake system has a simpleconstruction and which is capable of creating a strong turbulence in thecombustion chamber independently of the engine speed when an engine isoperating under a light load while ensuring a high volumetric efficiencywhen the engine is operating at a high speed under a heavy load.

According to the present invention, there is provided a multi-cylinderinternal combustion engine having a plurality of cylinders, each havinga combustion chamber and an intake valve which has a valve head, saidengine comprising: at least one mixture passage common to at least twocylinders and comprising a collecting portion having an inlet, and atleast two branch mixture passages branched off from said collectingportion, each of said branch intake passages having an upper wall and abottom wall and being connected said respective combustion chamber viasaid corresponding intake valve; fuel supply means arranged in the inletof said collecting portion; a common connecting channel; at least twochannel branches, each being connected to said common connecting channeland having an opening which opens into said respective branch intakepassage; and at least two rotatable throttle valves, each being arrangedin a respective one of said branch mixture passages at a locationupstream of the opening of a corresponding channel branch and having alower edge and an upper edge, said upper edge cooperating with the upperwall of said corresponding branch mixture passage to form therebetween amixture flow passage, the cross sectional area of which is increased asthe throttle valve is rotated in accordance with an increase in thelevel of the load of said engine, and the lower edge of each of saidthrottle valves cooperating with the bottom wall of said correspondingbranch mixture passage to prevent flow between the lower edge of saidthrottle valve and the bottom wall of said corresponding branch mixturepassage.

The present invention may be more fully understood from the descriptionof preferred embodiments of the invention set forth below, together withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a plan view, partly in cross-section, of an embodiment of anengine according to the present invention;

FIG. 2 is a cross-sectional side view taken along the line II--II inFIG. 1;

FIG. 3 is a cross-sectional view taken along the III--III in FIG. 2;

FIG. 4 is a cross-sectional side view of another embodiment according tothe present invention;

FIG. 5 is a cross-sectional plan view taken along the line V--V in FIG.4;

FIG. 6 is a plan view, partly in cross-section, of a further embodimentaccording to the present invention;

FIG. 7 is a cross-sectional side view of a still further embodimentaccording to the present invention; taken along the line VII--VII inFIG. 9;

FIG. 8 is a cross-sectional view taken along the line VIII--VIII in FIG.7;

FIG. 9 is a plan view, partly in cross-section, of the engineillustrated in FIG. 7; and

FIG. 10 is a graph showing changes in pressure in the intake portlocated at a position near the rear face of the valve head of the intakevalve.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, 1 designates an engine body; 2a, 2b, 2c, 2ddesignate No. 1 cylinder, No. 2 cylinder, No. 3 cylinder and No. 4cylinder, respectively; 3a, 3b, 3c, 3d designate intake valves; 4a, 4b,4c, 4d designate exhaust valves; 5a, 5b, 5c 5d designate intake ports;and 6a, 6b, 6d designate exhaust ports. Referring to FIG. 2, referencenumeral 7 designates a cylinder block, 8 a piston which is reciprocallymovable in the cylinder block 7, 9 a cylinder head fixed onto thecylinder block 7, and 10 a combustion chamber. A spark plug (not shown)is arranged in the combustion chamber 10.

Referring to FIGS. 1 and 2, a pair of carburetor housings 11, 12 ismounted on the engine body 1, and variable venturi type carburetorbodies 13, 14 are arranged in the carburetor housings 11, 12,respectively. Each of the mixture passages 15, 16 formed in thecarburetor housings 11, 12 is divided into two respective branch mixturepassages 17, 18 and 19, 20, and each of the branch mixture passages 17,18, 19, 20 is respectively connected to the intake ports 5a, 5b, 5c, 5d.Throttle valves 21, 22, 23, 24 of the carburetor bodies 13, 14 arearranged in the corresponding branch mixture passages 17, 18, 19, 20 andattached onto a common valve shaft 25. However, instead of beingattached onto the common valve shaft 25, the throttle valves 21, 22, 23,24 may be interconnected to each other by means of a link mechanism (notshown) so that the opening operation of all the throttle valves 21, 22,23, 24 is controlled at the same time. As is illustrated in FIG. 2, awedge shaped groove 32 is formed on the bottom wall of the branchmixture passage 18, and the throttle shaft 25 is arranged in the wedgeshaped groove 32. 6c, By positioning the throttle shaft 25 in the wedgeshaped groove 32, the flow-resistance which the mixture flowing in thebranch mixture passage 18 is subjected to becomes extremely small whenthe throttle valve 22 is fully opened, as illustrated by the broken linein FIG. 2. In addition, as is illustrated in FIG. 2, the carburetor body13 comprises a movable suction piston 26, a movable needle 27 and ametering jet 28. As is well known to those skilled in the art, thesuction piston 26 moves up and down so that the vacuum produced in themixture passage 15 located between the suction piston 26 and thethrottle valve 22 is maintained at a constant level.

A common connecting channel 29 extending in the longitudinal directionof the engine body 1 and having a cross-section which is smaller thanthat of the branch mixture passages 17, 18, 19, 20 is arranged beneaththe throttle valves 21, 22, 23, 24. In addition, four channel branches30a, 30b, 30c, 30d which are in communication with the common connectingchannel 29 and which have a cross-section smaller than that of thebranch mixture passages 17, 18, 19, 20 are formed in the cylinder head9, and the channel branches 30a, 30b, 30c, 30d open into the intakeports 5a, 5b, 5c, 5d at a position near the rear faces of the valveheads of the corresponding intake valves 3a, 3b, 3c, 3d, respectively.The openings of the channel branches 30a, 30b, 30c, 30d are directed tovalve gaps formed between the corresponding intake valves 3a, 3b, 3c, 3dand their valve seats when the intake valves 3a , 3b, 3c, 3d are opened,respectively.

FIG. 10 illustrates changes in pressure in the intake ports 5a, 5b, 5c,5d. In FIG. 10, the abscissa θ indicates crank angle, and the ordinate Pindicates pressure in the intake port in the vicinity of the rear faceof the valve head of the intake valve (hereinafter referred to as anintake port pressure). In addition, each of the reference lines A, B, C,D indicates the atmospheric pressure. Furthermore, in FIG. 10, thecurved lines E, F, G and H indicate changes in the intake port pressurein the intake ports 5a, 5b, 5c and 5d, respectively, and the arrows I,J, K and L indicate the opening duration of the intake valves 3a, 3b, 3cand 3d, respectively. Referring to the change in pressure in the No. 1cylinder shown in FIG. 10, the intake port pressure becomes a positivepressure over the range M of the crank angle immediately after theintake valve is opened, and then a vacuum is produced in the intake portof the No. 1 cylinder over the range N of the crank angle in which thepiston moves downwards.

After this, the intake port pressure again becomes a positive pressureover the range O of the crank angle after the piston begins to moveupwards. The change in the intake port pressure in the remainingcylinders is the same as that in the intake port pressure in the No. 1cylinder. Consequently, during the range P of the crank angle for theNo. 1 cylinder and No. 2 cylinder, shown in FIG. 10, a vacuum isproduced in the intake port of the No. 1 cylinder, while the intake portpressure of the No. 2 cylinder is positive. In addition, from FIG. 10,during the range Q of the crank angle for the No. 2 cylinder and No. 4cylinder, a vacuum is produced in the intake port of the No. 2 cylinderwhile, the intake port pressure of the No. 4 cylinder is positive;during the range R of the crank angle for the No. 3 cylinder and the No.4 cylinder, a vacuum is produced in the intake port of the No. 4cylinder while, the intake port pressure of the No. 3 cylinder ispositive, and during the range S of the crank angle for the No. 1cylinder and the No. 3 cylinder, a vacuum is produced in the intake portof the No. 3 cylinder while, the intake port pressure of the No. 1cylinder is positive. Consequently, referring to the No. 1 cylinder andthe No. 2 cylinder shown in FIG. 10, it will be understood that, in thefirst half of the intake stroke of the No. 1 piston, the mixture in theintake port 5b of the No. 2 cylinder is fed into the intake port 5a ofthe No. 1 cylinder via the channel branch 30b, the common connectingchannel 29 and the channel branch 30a due to the pressure differencebetween the vacuum in the intake port 5a and the positive pressure inthe intake port 5b. In the same manner, when the No. 2 piston is in theintake stroke, the mixture in the intake port 5d of the No. 4 cylinderis fed into the intake port 5b of the No. 2 cylinder via the channelbranch 30d, the common connecting channel 29 and the channel branch 30b;when the No. 4 piston is in the intake stroke, the mixture in the intakeport 5c of the No. 3 cylinder is fed into the intake port 5d of the No.4 cylinder via the channel branch 30c, the common connecting channel 29and the channel branch 30d; and when the No. 3 piston is in the intakestroke, the mixture in the intake port 5a of the No. 1 cylinder is fedinto the intake port 5c of the No. 3 cylinder via the channel branch30a, the common connecting channel 29 and the channel branch 30c.

FIG. 2 illustrates the case wherein the engine is operating under alight load. The throttle valve 22 is mounted so that a mixture flow gap33 is formed only between the upper edge of the throttle valve 22 andthe upper wall of the branch mixture passage 18. Consequently, the flowvelocity of the mixture formed in the carburetor body 13 increases asthe mixture converges towards the mixture flow gap 33. After the mixturepasses through the mixture flow gap 33 at high speed, the mixture flowsonly along the upper wall of the intake port 5b, still at high speed, asillustrated by the arrow A in FIG. 2, and then flows into the combustionchamber 10 through the valve gap formed between the intake valve 3b andits valve seat. In addition, as mentioned previously, the mixture isspouted from the channel branch 30b at the time of the intake stroke. Asis illustrated in FIG. 2, since the opening of the channel branch 30b isdirected toward the valve gap formed between the intake valve 3b and itsvalve seat, the velocity of the mixture passing through the valve gap,as illustrated by the arrow A in FIG. 2, is increased by the mixturespouted from the channel branch 30b. As a result, the mixture is causedto flow into the combustion chamber 10 at an extremely high speed. Atthis time, since the intake port 5b is tangentially connected to thecircumferential inner wall of the combustion chamber 10, as illustratedin FIG. 2, a strong swirl motion, shown by the arrow W in FIG. 1, iscreated in the combustion chamber 10. As a result of this swirl motionthe burning velocity is considerably increased, and a stable combustioncan thus be obtained.

When the engine is operating under a heavy load, since the throttlevalve 22 is fully opened, as illustrated by the broken line in FIG. 2,the mixture flows in the intake port 5b through the entire cross-sectionthereof. However, since at this time the mixture is spouted from thechannel branch 30b, a turbulence is created in the combustion chamber 10even when the engine is operating under a heavy load. In addition, as isillustrated in FIG. 2, it is preferable that a mixture stream guideplate 34 be arranged in the intake port 5b at a position near the upperwall of the intake port 5b. By thus arranging the guide plate 34, thecreation of the stream of mixture flowing only along the upper wall ofthe intake port 5b can be ensured when the engine is operating under alight load.

FIGS. 4 and 5 illustrate another embodiment according to the presentinvention. In this embodiment, the intake port 5b has a helical shape.In such a helically-shaped intake port, since the mixture flowing onlyalong the upper wall of the intake port 5b moves forward along a fixedflow path, as illustrated by the arrow Z in FIG. 5, a strong swirlmotion is created in the combustion chamber 10, both by the mixtureflowing along the above-mentioned fixed flow path and by the mixturespouted from the channel branch 30b.

FIG. 6 illustrates a further embodiment according to the presentinvention. In this embodiment, a single variable venturi type carburetorbody 36 having a construction which is the same as that of thecarburetor body 13 illustrated in FIG. 2 is arranged in a carburetorhousing 35. The outlet passage of the carburetor body 36 is divided intotwo mixture passages 37, 38, and each of the mixture passages 37, 38 isdivided into four respective branch mixture passages 17, 18, 19, 20.Each of the throttle valves 21, 22, 23, 24 is arranged in one of therespective branch mixture passages 17, 18, 19, 20. In this embodiment,there is an advantage in that the number of carburetor bodies can bereduced as compared with the case illustrated in FIG. 1.

FIGS. 7 through 9 illustrate a further embodiment according to thepresent invention in which an intake manifold 40 is fixed onto theengine body 1, and a carburetor 42 having a throttle valve 41 is mountedon the intake manifold 40. The intake manifold 40 comprises manifoldbranches 43, 44, 45, 46 which are connected to the intake ports 5a, 5b,5c, 5d, respectively. Secondly throttle valves 47, 48, 49, 50 arearranged in the outlets of the manifold branches 43, 44, 45, 46,respectively, and attached onto a common valve shaft 51. As isillustrated in FIg. 7, an arm 53 attached onto a valve shaft 52 of thethrottle valve 41 is interconnected to an arm 54 attached onto thecommon valve shaft 51 by means of a link 55, so that the secondarythrottle valves 47, 48, 49, 50 are gradually opened as the throttlevalve 41 is gradually opened. In addition, as is illustrated in FIG. 7,a cross-sectionally enlarged portion 44a is formed in the manifoldbranch 44, and the secondary throttle valve 48 is arranged in theenlarged portion 44a. The bottom wall 44b located at the end of theenlarged portion 44a forms a portion of a sphere, so that a mixture flowgap is not formed between the bottom wall 44b and the lower edge of thesecondary throttle valve 48 during the time the secondary throttle valve48 is rotated from the position illustrated by the solid line in FIG. 7to the position illustrated by the broken line in FIG. 7. Consequently,when the opening degree of the secondary throttle valve 48 is small, andthus the engine is operating under a light load, the mixture flow gap isformed only between the upper edge of the secondary throttle valve 48and the upper wall of the enlarged portion 44a. As a result, the mixtureflows only along the upper wall of the intake port 5b. In thisembodiment, the changes in pressure produced in the intake port at aposition near the rear face of the valve head of the intake valve are asshown in FIG. 10. Consequently, since the mixture is spouted from thechannel branches 30a, 30b, 30c, 30d into the combustion chamber 10 at ahigh speed, a strong swirl motion is created in the combustion chamber10. In addition, in either of the above-described embodiments, it ispossible to recirculate the exhaust gas into the common connectingchannel 29.

As is illustrated in FIGS. 1, 2, 4, 6, 7, and 9, by positioning thethrottle valves 21, 22, 23, 24 at the outlets of the branch mixturepassages 17, 18, 19, 20 and by positioning the secondary throttle valves47, 48, 49, 50 at the outlets of the manifold branches 43, 44, 45, 46,the positive pressure which is caused by blowing the mixture back intothe intake port is maintained without being attenuated. Since thepressure difference between this positive pressure and the vacuum whichacts on the openings of the channel branches 30a, 30b, 30c, 30d ismaintained at a high value for a long time, it is possible to produce anextremely strong swirl motion in each combustion chamber 10. Inaddition, since the mixture flows from the intake port of a givencylinder into the intake port of the other cylinder via the commonconnecting channel 29, the mixing operation of the mixture is improved,and the distribution of fuel to each cylinder becomes uniform.

According to the present invention, by causing the mixture to flow onlyalong the upper wall of the intake port and increasing the flow velocityof that mixture by the mixture spouted from the channel branch, it ispossible to produce a strong swirl motion in the combustion chamber whenan engine is operating under a light load. As a result, the burningvelocity can be increased independently of the engine speed when anengine is operating under a light load while ensuring a high volumetricefficiency when the engine is operating at a high speed under a heavyload.

While the invention has been described with reference to specificembodiments chosen for purposes of illustration, it should be apparentthat numerous modifications could be made thereto by those skilled inthe art without departing from the spirit and scope of the invention.

What is claimed is:
 1. A multi-cylinder internal combustion enginehaving a plurality of cylinders, each having a combustion chamber and anintake valve which has a valve head, said engine comprising:at least onemixture passage common to at least two cylinders and comprising acollecting portion having an inlet, and at least two branch mixturepassages branched off from said collecting portion, each of said branchmixture passages having an upper wall and a bottom wall and beingconnected to said respective combustion chamber via said correspondingintake valve; fuel supply means arranged in the inlet of said collectingportion; a common connecting channel; at least two channel branches eachbeing connected to said common connecting channel and having an openingwhich opens into one of said respective branch mixture passages; atleast two rotatable throttle valves each being arranged in a respectiveone of said branch mixture passages at a location upstream of theopening of a corresponding channel branch and having a lower edge and anupper edge, said upper edge cooperating with the upper wall of saidcorresponding branch mixture passage to form therebetween a mixture flowpassage, the cross sectional area of which is increased as the throttlevalve is rotated in accordance with an increase in the level of the loadof said engine, the lower edge of each of said throttle valvescooperating with the bottom wall of said corresponding branch mixturepassage to prevent flow between the lower edge of said throttle valveand the bottom wall of said corresponding branch mixture passage whensaid throttle valve is at least partly open; and a guide plate arrangedin each of said branch mixture passages at a position located downstreamof and near a corresponding one of said throttle valves in the vicinityof the upper wall of said branch mixture passage.
 2. A multi-cylinderinternal combustion engine having a plurality of cylinders, each havinga combustion chamber and an intake valve which has a valve head, saidengine comprising:at least one mixture passage common to at least twocylinders and comprising a collecting portion having an inlet, and atleast two branch mixture passages branched off from said collectingportion, each of said branch mixture passages having an upper wall and abottom wall and being connected to said respective combustion chambervia said corresponding intake valve; fuel supply means arranged in theinlet of said collecting portion; a common connecting channel; at leasttwo channel branches each being connected to said common connectingchannel and having an opening which opens into one of said respectivebranch mixture passages; at least two rotatable throttle valves eachbeing arranged in a respective one of said branch mixture passages at alocation upstream of the opening of a corresponding channel branch andhaving a lower edge and an upper edge, said upper edge cooperating withthe upper wall of said corresponding branch mixture passage to formtherebetween a mixture flow passage, the cross sectional area of whichis increased as the throttle valve is rotated in accordance with anincrease in the level of the load of said engine, the lower edge of eachof said throttle valves cooperating with the bottom wall of saidcorresponding branch mixture passage to prevent flow between the loweredge of said throttle valve and the bottom wall of said correspondingbranch mixture passage when said throttle valve is at least partly open;and mixture guide means arranged in each of said branch mixture passagesat a position located downstream of and near a corresponding one of saidthrottle valves for ensuring that the mixture flows only along the upperwall of each branch mixture passage when the engine is operating under alight load.
 3. A multi-cylinder internal combustion engine as claimed inclaim 2, wherein said common connecting passage has a cross-sectionwhich is smaller than that of said branch mixture passage.
 4. Amulti-cylinder internal combustion engine as claimed in claim 2, whereineach of said channel branches has a cross-section which is smaller thanthat of said branch mixture passage.
 5. A mulit-cylinder internalcombustion engine as claimed in claim 2, wherein each of said channelbranches has a helical shape in the vicinity of a corresponding one ofsaid intake valves.
 6. A multi-cylinder internal combustion engine asclaimed in claim 2, wherein each of said throttle valves is arranged ata position near a corresponding one of said intake valves.
 7. Amulti-cylinder internal combustion engine as claimed in claim 6, whereinsaid at least one mixture comprises at least one carburetor housingforming therein said at least two branch mixture passages each having anoutlet which is connected to a respective combustion chamber via acorresponding intake valve, each of said throttle valves being arrangedin said outlet of said respective mixture passage.
 8. A multi-cylinderinternal combustion engine as claimed in claim 2, wherein the opening ofeach of said channel branches is arranged in the vicinity of the valvehead of a corresponding one of said intake valves.
 9. A multi-cylinderinternal combustion engine as claimed in claim 8, wherein the opening ofeach of said channel branches is directed to a valve gap formed betweensaid corresponding intake valve and a valve seat thereof when saidintake valve is opened.
 10. A multi-cylinder internal combustion engineas claimed in claim 2, wherein each of said throttle valves is attachedonto a throttle shaft arranged on the bottom wall of a corresponding oneof said branch mixture passages, the lower edge of each of said throttlevalves cooperating with the bottom wall of said corresponding branchmixture passage to always prevent the mixture from flowing between thelower edge of said corresponding throttle valve and the bottom wall ofsaid corresponding branch mixture passage.
 11. A multi-cylinder internalcombustion engine as claimed in claim 10, wherein said throttle valvesare attached onto a common throttle shaft.
 12. A multi-cylinder internalcombustion engine as claimed in claim 10, wherein a wedge shaped grooveis formed on the bottom wall of each of said branch mixture passages,the throttle shaft for each valve being arranged in the correspondingone of said wedge shaped grooves.
 13. A multi-cylinder internalcombustion engine as claimed in claim 2, wherein said engine furthercomprises another throttle valve arrange in said mixture passage, saidthrottle valves being operatively connected to said other throttle valvefor increasing the opening degree of said throttle valves in accordancewith an increase in the opening degree of said other throttle valve. 14.A multi-cylinder internal combustion engine as claimed in claim 13,wherein said throttle valves are mechanically connected to said otherthrottle valve by means of a link mechanism.
 15. A multi-cylinderinternal combustion engine as claimed in claim 13, wherein said at leastone mixture passage and at least two branch mixture passages comprise atleast one intake manifold having at least two outlets, each of saidthrottle valves being arranged in a respective one of the outlets ofsaid intake manifold.
 16. A multi-cylinder internal combustion engine asclaimed in claim 2, wherein each of said throttle valves is attachedonto a throttle shaft extending through a center of said respectivebranch mixture passage, the lower edge of each of said throttle valvescooperating with the bottom wall of a corresponding one of said branchmixture passages to prevent the mixture from flowing beteen the loweredge of said throttle valve and the bottom wall of said correspondingbranch mixture passage when the opening degree of each of said throttlevalves is below a predetermined value.
 17. A multi-cylinder internalcombustion engine as claimed in claim 16, wherein said throttle valvesare attached onto a common throttle shaft.
 18. A multi-cylinder internalcombustion engine as claimed in claim 16, wherein each of said branchmixture passages comprises a portion having a first diameter, a portionhaving a second diameter larger than said first diameter, and aconnecting portion connecting said first diameter portion to said seconddiameter portion, each of said throttle valves being arranged in acorresponding one of said second diameter portions and cooperating withthe bottom wall of a corresponding connecting portion at the lower edgeof said throttle valve.
 19. A multi-cylinder internal combustion engineas claimed in claim 18, wherein the bottom wall of each connectingportion forms a portion of a sphere.